This invention relates to a heat-sensitive recording composition and a recording sheet prepared by use of said composition. More particularly, the invention pertains to a heat-sensitive recording composition comprising a normally colorless or pale-colored chromogenic substance, a phenolic substance capable of developing the color of said chromogenic substance at an elevated temperature, and a water-soluble binder, characterized by containing as a hardener for said binder a compound having 2 to 10 1,2-epoxy ring structures in one molecule, and to a recording sheet prepared by use of said composition.
Heretofore, the fact that a chromogenic substance such as Crystal Violet Lactone, for example, reacts with a phenolic substance to form a color has been well known according to O. Fischer and F. Romer: Berichte der Deutschen Chemishen Gesellschaft, Vol. 42, pages 2934 - 2936 (1909). Further, applications of the above-mentioned reaction to heat-sensitive recording sheets have also been well known according to, for example, Japanese patent publication No. 14,039/70.
The heat-sensitive recording sheet of the present invention, in which are used a normally colorless of pale-colored chromogenic substance and a phenolic substance, is prepared by individually dispersing fine particles of the above-mentioned two substances in an aqueous medium containing a binder, mixing the resulting dispersions with each other, adding to the mixed dispersion a compound having 2 to 10 1,2-epoxy ring structures, and coating the resulting composition on a support, followed by drying.
As a binder for use in a heat-sensitive recording composition comprising the above-mentioned chromogenic substance and phenolic substance, there may be shown polyvinyl alcohol, methyl cellulose, hydroxyethyl cellulose, gum arabic, gelatin, styrene-maleic anhydride copolymer or acrylic resin. However, a heat-sensitive recording sheet prepared by coating the said composition by use of the above-mentioned water-soluble binder has no water resistance at all, and when water is spilt or dropped on the recording sheet, the surface coating is undesirably flowed away or stained. Accordingly, it is necessary to harden the coating layer to make the recording sheet water-resistant.
As crosslinking hardeners for the above-mentioned water-soluble binders, there have been known formalin, glyoxal, chromium alumn and melamine-formalin resin. These hardeners, however, have such drawbacks that they crosslink during preparation of coating liquids, thereby agglomerating the coating liquids to make them uncoatable, they have little hardening effects, and they bring about marked coloration in texture of recording sheets.